1. Field of the Invention
The present invention relates to a PWM generating unit for generating a PWM (Pulse Width Modulation) signal according to a duty value, an image forming apparatus, an image forming method, and an image forming system.
2. Description of the Related Art
In a prior art image forming apparatus using the electrophotography method, a toner image is adhered on a photoconductive body by utilizing an electric field caused by a potential difference between a developing roller and the photoconductive body. It is generally known that this electric field varies depending on a developing gap, that is, the distance between the photoconductive body and the developing roller. The variation in the developing gap may be generated due to rotary fluctuation of the photoconductive body or rotary fluctuation of the developing roller. The rotary fluctuation may be caused by at least one of an unstable rotation of a motor or the like that rotates the photoconductive body or the developing roller, an eccentricity of the photoconductive body or the developing roller, an error in a mounting position of the photoconductive body or the developing roller, and the like. When the developing gap varies, non-uniformity of a density of the image (hereinafter referred to as “density non-uniformity”) occurs.
Because the density non-uniformity of the image due to the variation in the developing gap is caused by the rotary fluctuation of the photoconductive body or the rotary fluctuation of the developing roller, the density non-uniformity of the image occurs periodically and may easily be confirmed visually. Hence, in the prior art, measures are taken to suppress the density non-uniformity of the image due to the variation in the developing gap.
For example, a Japanese Laid-Open Patent Publication No. 9-62042 proposes a technique to comprehensively reduce stripe shaped density non-uniformity generated periodically in the image, in an image forming apparatus employing the electrophotography method or the static recording method. In addition, a Japanese Laid-Open Patent Publication No. 2007-60865 proposes a technique to correct a rotational speed of a rotary body by detecting the variation in the rotational speed of the rotary body.
However, the prior art may store density variation data for every image forming condition in a storage unit, for example, and correct the density non-uniformity using the stored density variation data. According to this method, in the case of a full-color image forming apparatus, for example, the density variation data are required for every color, and the storage unit needs to have a large storage capacity.
In addition, the prior art may read the density variation data within a short time during the image formation, and carry out a process by a CPU (Central Processing Unit) to correct the density non-uniformity. Consequently, a processing load on the CPU increases, and in some cases, a dedicated CPU may be required exclusively for the correction of the density non-uniformity.